Drilling operators typically need to know the time required for cuttings made by drilling operations to reach the surface, usually known as lag time. The lag period is typically either measured as a function of time or as a number of drilling fluid (frequently known as “mud”) pump strokes. This has often been calculate by performing what is known as a carbide lag test, in which a small paper packet containing calcium carbide is inserted into the drill string when the Kelly is unscrewed from the pipe to make a connection. The time of the insertion of the carbide is noted, along with the pump stroke count on the mud pump. Once the connection is completed, drilling resumes. The packet is entrained in the mud and moves downhole with the mud. The mud then breaks down the paper casing, allowing the calcium carbide to react with the mud, generating acetylene gas. The gas circulates with the drilling mud downhole and back up the annulus of the well, until it reaches the surface. The acetylene can then be detected at the gas trap of the mud system, causing a peak or spike in gas readings for acetylene. The time and pump stroke count corresponding to the peak may then be noted again, and a lag interval calculated.
Carbide tests require assistance from the rig crew, and introduce time, technical, and safety issues, thus are generally not performed at every connection of drill pipe to the drill string. In some areas, carbide tests are regulated or even prohibited because of those technical and safety issues.
Certain conditions in a well, however, may affect the accuracy of a lag time calculation. One of these conditions is caving.
When drilling a well, sections of the well may be enclosed in casing, but other sections, known as open hole sections, have no casing, either because the well is being drilled without casing or because the casing has not yet been run into that section of the well. Open hole drilling is frequently done in horizontal or other directional drilling operations.
While drilling an open hole section, there is a difference between the theoretical open hole and the actual one, including changes in the borehole volume caused by cavings or cuttings accumulations. This difference may significantly affect the lag time reported. In addition, a correct estimation of the open hole volume would be valuable for other rig operations, including cementing, mud displacements, hydraulics, etc.